Prior art devices relating to limited slip differential mechanisms of the speed-sensitive type are well known. In particular, it is known to provide a fluid pump structure within the differential mechanism which will operate in a speed sensitive manner to resist relative rotation. Many of the known prior art devices incorporate pumping structures and a means to block the output of the pump in response to speed difference which tends to resist relative differential action. These devices have proved unsatisfactory in that the relative rotation does not seem to provide sufficient pumping action such that the blocking of the outlet is effective to retard differential action except in cases of extremely high speed difference. Further prior art devices are known having speed-sensitive fluid pump mechanisms, but each of these devices tend to have relatively complicated structures which are expensive to manufacture and take up substantial space within the differential mechanism to add the speed-sensitive feature.